Electric carrier wave signaling system



Dec.. 19, 1944. B. B. JAcoBsEN ELECTRIC CARRIER WAVE SIGNALING SYSTEM Filed Dec. 31. 1941 2 Sheets-Sheet l ATTORNEY Dec. 19, 1944. B. B. JAcoBsEN ELECTRIC CARRIER WAVE SIGNALING SYSTEM 2 Sheets-Sheet 2 Filed Dec. 5l, 1941 Patented Dec. 19, 1944 nnEc'rnic @IER WAVE rSIGNALING sYs'rEM Bent Blow Jacobsen, London, England, assigner to International Standard Electric Corporation,

New York, N. Y.

Application December s1, i941, serial No. 425,177

Great Britain January 24, 1941 laims.

This invention relates to electric carrier wave signaling systems, and in particular to arrangements for maintaining in operation at least some of the channels of multi-channel open wire systems during exceptionally bad weather conditions.

The attenuation of an open wire line at carrier frequencies may increase very considerably during bad weather, and the attenuation changes are normally compensated by well known automatic gain control arrangements. rangements, however, cannot compensate forthe increase of line noise which is the consequence of the high receiving gain required when the line attenuation is high. Moreoven'when planning the repeater spacing and power capacity in an open wire carrier system, the bad weather attenuation increase can only be anticipated to a limited extent. Under conditionsv of ice and snow, the line attenuation may rise almost indeiinitely. The limit is probably set by the amount of ice or snow which the wires will carry without breaking, and it is desirable to maintain at least some of the channels of the carrier system in operation, if possible, under all conditions for which the line does not actually break.

The worst conditions of ice and snow occur only very rarely, and it would be uneconomical to design the number of repeater stations and their maximum output power capacity for such conditions: They will be accordingly designed to Such arcrease in transmission level will however be limited by the corresponding increase in inter-modulation noise due to the higher level in the terminal amplifiers and line repeaters. The maximum level increase will be that which makes the inter-modulation noise approximately equal to the line noise.

In the case of a twelve-channel open wire carrier system, the transmitting level may be increased, and the circuit noise decreased, by about 9 decibels, if the number of channels in operation is reduced to three; or by about 13 decibels if only one channel is used.

If the main cause of distortion due to increase of transmitting level is non-linearity in the amplifiers, then some further increase in the level will be possible if thel channels to be left in operation are suitably selected; Ii', however, the distortion is chiefly due to overloading the ampliflers, then no advantage can be obtained by selection..

operate for a reasonable range of weather variabad weather:

(1) An increase in the transmitting level combined with an equal decrease in the receiving gain.

(2) The use of severe peak-voltage limitation of the incoming speech waves.

(3) Reduction in the number of channels in operation. u

(4l Selection of the channels to be kept in operation for minimum inter-modulation in the ampliiier.4

The last three expedients all operate to allow Excessive limitation of the peak voltage will introduce distortion into the received speech, but the effect will be comparable with the eiect of the line noise. Voltage limitation will, however, permit some increase in the transmitting level, and the voltage should be limited 'until the disturbing effect of the distortion introduced is approximately equal to the disturbing effect of the line noise.

The maximum increase in transmitting level obtainable by all these methods,` including the reduction of the number of channels to one, is approximately 30 decibels.

In general, the level changes described above would be made in such a way that the pilot frequencies used for the automatic gain control are not affected. In some cases, however, it may also be desirable to raise the pilot level at the sending end in order that it may be suiliciently above the line noise at the receiving end. In such cases it would be necessary also to introduce means for reducing the sensitivity of the automatic gain control at the repeater stations, in order that the desired control may be obtained from the higher pilot level.

According to the invention, in a carrier wave transmission system for operation over a communication path, means is provided for reducing the noise level received in any channel under exceptional conditions, the said means comprising means for limiting the voltage of the speech Waves incoming to one or more of the channels, and means for raising the level oi' the channel sidebands' transmitted to the said path, while maintaining substantially constant the overall transmission equivalents of the channels. In addition, means may also be provided for taking out of service one or more of the channels, and the channels which remain may be so selected that the eiect of inter-modulation in the ampliers is a minimum.

The invention will be more clearly understood from the following detailed description referring to the accompanying drawings, which show a twelve channel carrier system, to which are added components according to the invention, and in which:

Fig. ishows the transmitting arrangements at the east terminal of the system;

Fig. 2 shows an intermediate repeater; and

Fig. 3 shows the receiving arrangements at the west terminal of the system.

In these diagrams, the components according to the invention which areadded to the system are shown in dotted outlines.

The east and west terminals shown in Figs. 1 and 3 are supposed to be connected respectively by sections of line LE-LE and LW-LW to an gterzmediate repeater station represented by Referring to Fig. l, speech currents from twelve local lines. L1 to L12 corresponding respectively to the twelve channels (of which only two are shown complete), pass through twelve corresponding four-wire terminating sets T1 to T11 and are d1- rected to the channel modulators M1 to M12. 'Ihey then pass through the channel filters TF1 to TF1: which select the required sidebands. These filters each cover a range of frequencies characteristic of a particular channel, and their outputs are connected in parallel, as indicated, to the frequency changing equipment FC in which the channel frequency bands are transposed to the range of frequencies used for transmitting from east to west. The transposed sidebands are amplified by the transmitting ampiier TA and reach the line LE through the transmitting line filter LFW. The receiving line filter LFE is used for conveying the currents received at the east terminal and leads to the receiving equipment (not shown) at RE. The lter LFW blocks the received currents and the filter LFE blocks the transmitted currents.

The line consists of several sections connected by repeater stations; actually only two such sections are shown, LE--LEand LW-LW, which connect the repeater station shown in Fig, 2, respectively with the east terminal Fig. 1, and with the west terminal, Fig. 3. Any other repeater stations there may be can be supposed to be similar to Fig. 2. The usual type of repeater is shown, 4consisting of two branches one of which contains an amplifier AW for amplifying the currents transmitted from the east to the west terminal, and the other contains a similar amplifier AE for the reverse direction. Separating lin'e filters LFW and LFE are provided as shown, which may be similar to those used at the east terminal Fig. l. Each amplifier is provided with a gain control arrangement which may comprise. for example, a

filter PF as shown, connected to the output of the amplifier for selecting the pilot frequency or frequencies, leading to a gain control unit GU which adjusts the amplier gain according .to the vvariations of the line equivalent, in the well known manner. l

The transmitted currents from the east terminal Fig. 1 are accordingly amplified by the amplifier AW in Fig. 2, and owing to the gain control the output levels of the sideband frequencies will be substantially independent of changes in the line equivalent over a wide range up to some maximum value of the line loss. After amplification, the side bands are transmitted by the line section LW-LW to the west terminal where they are directed by the line lters LFW and LFE (which may be similar to those used at the east terminal) to the receiving amplifier, which is provided with automatic gain control equipment PF and GU as described in connection with Fig. 2. The side bands then pass to the frequency changing equipment FC similar to that used in the east terminal where they aretranslated to a range suitable for demodulation, and distributed to the appropriate channels by ,the receiving filters RF1 to RFiz. Demodulation occurs respectively in the demodulators D1 to D12, and after amplication in the channel ampliilers CA1 to CAn, the speech currents are directed to the local lines L1 to L12 by the four-wire terminating sets T1 to Tm. As in the case of Fig. 1, only two channels are shown complete in Fig. 3.

lt will be understood that transmission from the west terminal to the east terminal takes place in a manner similar to that just described, except that a different range of frequencies will be used ior transmission over the lines LE-LE and LW-LW. A

The arrangements which have been described up to the present are well known. The present invention consists in the application of certain additional components shown in dotted outlines in Figs. l and 3, which will now be explained.

In Figs. 1 and 3, units i and 2 are respectively shown interposed between the frequency changer FC and the corresponding amplifier TA or RA. The unit i consists of an attenuating pad AP. and two change-over switches 3 and i by means of which the pad may be switched into or out of the circuit. The unit 2 comprises a similar pad AP with two switches A5 and t. The attenuation introduced by the pads AP is the same for both the units, which diier only in that the normal positions of the switches are such that the pad AP is in circuit in the unit i and out of circuit in unit 2.

The attenuation of the pads AP will be made equal to the increase in transmitting level which is required during the incidence of exceptional weather conditions; and the gain of the amplier TA in Fig. l will be so adjusted that the normal level is transmitted to the line LE under ordinary conditions. The normal level will accordingly ybe received from the line LW in Fig. 3. When the exceptional conditions apply switches 3; fi, 5 and 6 will be operated (automatically or by hand as will be explained below) so that the pad AP will be cut out at the transmitting end, and inserted at the receiving end. This will raise the level of the sidebands in the lines LE-LE and 11W-LW, while maintaining the equivalent between the frequency changers FC in Figs. 1 and 3 at the same value.

As hasalready been explained, when the line attenuation is high (as during severe weather may be carried out simultaneously. Alternativeconditions) the noise in the channels at the receiving end (Fig. 3) is mainly line noise and accordingly the overall noise levelwill be reduced practically by the attenuation introduced by the unit 2, which may be from to'30 decibels-according to the number o! channels taken out-of -service during bad weather.

In Fis. l, a voltage limiter unit 'I is shown connected to the input side o! the modulator M1 oi' channel No. l. This unit comprises a peak volt.. age limiter l of any suitable type together with a switch t, normally open, ior connecting the limiter s in circuit. In-Fig. l also is shown a switch unit I0 connected to channel No. 12, and comprising a switch Ii used for disconnecting the channel from the local line Ln, and connecting it'to a terminating impedance conventionally shown at l2 in order to maintain the balance of the four-wire terminating set Tis. Channel I2 at the receiving end in Fig. 3 is provided with a similar switch unit Il comprising a switch I4 and a terminating impedance ill.

All channels which are to be taken out of serv ice during severe weather may be provided with switch units similar to Iii and i3, and those remaining may be provided with voltage limiter units similar to il. The peak voltage limitera 9 will be adjusted to restrict the voltage peaks of the incoming speech waves as much as possible without degrading too much the received speech.

As already mentioned, the voltage may be limitedA until the disturbing effect of the distortion produced by the limitation at the receiving endis approximately the same :as the disturbing effect produced by the line noise. For example, the limiters t might be adjusted so that the instantaneous voltage across the terminals does not excced the voltage produced at the same point when a subscriber transmits l milliwatt into his line. By reducing as far as possible the number oi' channels kept in service andby limiting as strictly as possible the voltage peaks by means oi' the limiter units l', the maximum increase in transmitting level can be realised.l

A further increase in transmitting level can be obtained by keeping in service those channels for which the eilect ci inter-modulation due to nonlinearity of the amplifiers `is a minimum. However, as already mentioned no advantage can be gained by such selection of the channels if the intermodulation is due to overloading of the ampliiiers.

It will be understood, of course, that a similar pair of units 2 and I (not shown) will be provided for transmission in the opposite direction.

and will be located respectively in the paths indicated at RE and TE in Figs. l and 3. Likewise those channels kept in service will be provided with voltage limiter units (not shown) connected at corresponding points in Fig. 3.

In order to facilitate changing over the circuit, the switches 3, l, d and Il in Fig. l may be pro vided by means of spring groups on a single key, or relay, or on separate relays intended to be operated in series or in parallel. Similarly the switches 5, B and I4 may be arranged for simultaneous operation.

The simplest Aarrangement will be for the switching to be done by hand by the operators, when it is considered that conditions have become sufficiently bad. For example, the operator at the west terminal may decide that the received noise level has increased beyond theallowable limit and may then advise the operator at the east terminal, so that switching at the two ends ly, however. the switching at the west terminal may be controlled automatically from the east terminal. In this case a supervisory current would be connected at the east terminal, Fig. 1, at point Il (or any point before unit I) and would be transmitted to the west terminal and accepted by a illter (not shown) connected at point I1, and passed to a detecting device. When the operator at the east terminal cuts out the pad AP in the unit I, the increase in level of the supervisory current received at.. the west terl minal, Fig. 3. may be made to cause a relay to be operated which in turn causes switches i, I and Htc be directly or indirectly operated. Al ternatively, the switching may be entirely automatic, and controlled by the noise level received at the west terminal. For this purpose, an additional filter would be connected at the point ii in Fig. 3, adapted to select frequencies not used for the channel sidebands. This filter would be connected to a detector for detecting the noise, which would operate a relay whenthe noise level rises above a certain limit, for sending a supervisory'tone to the east terminal where it would be detected by a filter and detector similar to those Vconnected at point I'I in the west terminal, Fis. 3, Just described. The west terminal may be switched either directly by the noise detector, or indirectly by a supervisory tone from the east terminal as explained. With this arrangement it is desirable that the nose detector should have a slow response so that the switching does not take place unless the severe noise condition persists for a minute or more, for example.

It will usually be desirable that the pilot currents used for the automatic gain control arrangements in the system should not be ail'ected by the change in transmission level. These will accordlngly be introduced at the point I 8 in the east terminal, Fig. 1, after the unit i. Ii however, the received noise is excessive, it may be desirable or necessary to raise the level o f the pilot currents as well as that of the sdebands in had weather.

For this purpose the pilot currents may be introduced at the east terminal before the unit i, at point I9, for example, or ii a different level change is required for the pilot currents, they may be connected to point I8 through an extra switching unit similar to unit I but introducing some other value of attenuation. must also be made at the repeater stations and at the west terminal for changing the sensitivity of the gain control arrangements to operate on the new level of the pilot current.

The maximum change in sideband level on the line, and the corresponding maximum reduction in the received noise level in severeweather conditions, may range from about 10 decibels when all channels are kept in service and are provided with voltage hunters, to about 30'decibels when only one channel is kept in service. 'How many channels can be maintained in operation clearly depends on the severity of the line noise under the worst conditions. In order to keep as I many channels as possible working for as long as possible it might be useful to reduce the number of channels in two or more steps, in which case some channels might be provided with switching units III and I3 as well as with voltage limiter `units 1.

Arrangements 1. A carrier wave transmission system comprising a plurality of speech channels, a carrier wave transmission path, a plurality of low frequency lines over which speech waves are transmitted, means for transmitting carrier waves over said transmission path modulated in accordance with the speech waves in each of said speech channels, a peak voltage limiter, switching means for con necting said peak voltage limiter in circuit to limit the voltage of speech waves incoming over at least one of said low frequency lines, an attenuating pad in the path of said carrier waves and switching means for disconnecting said attenuating pad from the path of the modulated carrier waves at the transmitting terminal of the system, whenever said peak voltage limiter is connected in circuit, whereby a higher level or carrier. transmission can be used without undue overloading.

2. A carrier wave transmission system comprising a plurality of speech channels, a carrier wave transmission path, a plurality of low frequency lines over which speech waves are transmitter, means for transmitting carrier waves over said transmission path modulated in accordance with the speech waves in each of sa'id speech channels, means for limiting the voltage Peaks of the speech waves incoming over a predetermined number of said low frequency lines, means for switching saisiv peak voltage limiting means into the circuit, a first attenuating pad, switching means 'for connecting said first pad -in or disconnecting seid nr'st pad from the path of the modulated carrier waves at the transmitting terminal of the syst foi a second similar attenuating pad and switching means for disconnecting said second pad from the path of the modulated carrier waves at the receiving terminal of the system when said first pad is connected in the path of the modulated carrier waves at the transmitting. terminal, and means for connecting said second pad in the path of the modulated carrier waves at the receiving terminal when said iirst pad is disconnected from the path of the modulated carrier waves at the transmitting terminal, said rst pad being disconnected and said second pad connected whenever said peak voltage limiting means is inserted into the circuit, whereby a higher level of carrier transmission can be usedwithout undue over loading.

3. System according to claim l, and also including terminating impedances and switching means for connecting said terminating impedances to the particular low frequency lines into which said peak voltage limiting means are not connected when raising the carrier transmission level, whereby onlya predetermined number of channels are used during said higher level transmission, channels not used are suitably balanced by said terminating impedances, and cross modulation between channels is substantially reduced.

BENT BLoW JAcoBsnN. 

